1207 
In the RöÖNrGreNogram *) of the sphalerite: ZnS, which crystallizes 
in the hexacistetrahedrical class, no other symmetry could be stated 
than that which corresponds to the hexacis-octahedrical crystals. 
On the other hand there is certainly another symmetry present 
in the case of pyrite’): FeS, which belongs to the dyacisdodeca- 
hedrical class, and which possesses thus a centre of symmetry. 
The right-handed and left-handed rotating crystals of sodiumchlorate : 
NaClO, gave, on transmission of a pencil of R6nreuNrays, identical 
patterns (Table I, fig. 1 and 2), which, if the plates were parallel 
to the faces of the cube, were of the same apparent symmetry, as 
the images of the pyrite, — just as Ewarp and Friepricn’) have 
also stated. It can be easily shown, that the same erystal (dextro- 
gyratory), if radiated through perpendicularly to each of the cube- 
faces, always gave the same image, absolutely corresponding with 
that of a laevogyratory crystal, under the same conditions of expe- 
riment. Thus the absence of a symmetry-centre in this case could 
not be stated; both the symmetry-planes, perpendicular to each other, 
which in the case of the pyrite can be attributed to the crystals 
themselves, appear here in the RONTGENogram, because the symmetry- 
centrum of the radiation is superimposed on the symmetry of the 
chlorate-crystals, which symmetry is characterised by the presence 
of only three perpendicular binary axes and the four ternary ones *). 
According to Frieper, plates of dextro- and laevogyratory quartz, 
if cut parallel to {OOO1}, will give ¢dentical ROnreEN-patterns. Just 
in the same way, the crystals of dextro- and laevogyratory /uteo-tiiaethy- 
lenediamine-kobaltibromide *) : {Co(Aein),}Br,-+-2H,O, which erystallizes 
in the tetragonal system, gave identical R6OnTGEN-patterns, showing 
the presence of four vertical planes of symmetry. 
1) Laur, Frieprica und Knippine, Bayr. Ak. der Wiss. München, 303, (1912). 
2) It is remarkable, that the spots have not an oval, but a rectangular shape here; 
this fact cannot be explained in the way suggested by Brace, by the incomplete 
parallellism of the incident rays of the pencil. Such phenomena were observed 
likewise with some other crystals, so eg. with sylvine (KCl), perpendicular to 
the ternary axis. With sy/vine also perpendicular to a quaternary axis the rectan- 
gular spots were observed ; moreover the central spot here shows a radiation in 
eight directions, parallel to the faces of the rhombusdodecahedron, a phenomenon 
quite unexplicable at this moment. As to the rectangular shape of the spots, we 
are persuaded now that it is principally connected with the thickness of the 
crystalplates: the phenomenon manifests itself only in the case of thick plates, 
being more prominent, if the plate is thicker, 
3) Ewatp und Friepricu, Ann. der Phys. (4), 44, 1183 (1914); vide also; Friepricu, 
Deutsche Naturforscher und Arztetag, Wien, (1913); Brace, Proc. R. Soc. London, 
89 A, 477, (1914). 
4) F. M. Jaecer, Verslagen Kon. Ak. Amst. April, (1915). (Still to be translated 
in these Proceedings). . 
